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  <section id="how-varnish-met-cheri-6-n">
<span id="phk-cheri-6"></span><h1>How Varnish met CHERI 6/N<a class="headerlink" href="#how-varnish-met-cheri-6-n" title="Link to this heading">¶</a></h1>
<section id="varnish-socket-addresses">
<h2>Varnish Socket Addresses<a class="headerlink" href="#varnish-socket-addresses" title="Link to this heading">¶</a></h2>
<p>Socket addresses are a bit of a mess, in particular because nobody
dared shake up all the IPv4 legacy code when IPv6 was introduced.</p>
<p>In varnish we encapsulate all that uglyness in a <code class="docutils literal notranslate"><span class="pre">struct</span> <span class="pre">suckaddr</span></code>,
so named because it sucks that we have to spend time and code on this.</p>
<p>In a case like this, it makes sense to make the internals strictly
read-only, to ensure nobody gets sneaky ideas:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>struct suckaddr *
VSA_Build(void *d, const void *s, unsigned sal)
{
    struct suckaddr *sua;

[… lots of ugly stuff …]

return (RO(sua));
}
</pre></div>
</div>
<p>It would then seem logical to use C’s <code class="docutils literal notranslate"><span class="pre">const</span></code> to signal this fact,
but since the current VSA api is currently defined such that users
call <code class="docutils literal notranslate"><span class="pre">free(3)</span></code> on the suckaddrs when they are done with them, that does
not work, because the prototype for <code class="docutils literal notranslate"><span class="pre">free(3)</span></code> is:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>void free(void *ptr);
</pre></div>
</div>
<p>So you cannot call it with a <code class="docutils literal notranslate"><span class="pre">const</span></code> pointer.</p>
<p>All hail the ISO-C standards committee!</p>
<p>This brings me to a soft point with CHERI: Allocators.</p>
</section>
<section id="how-to-free-things-with-cheri">
<h2>How to free things with CHERI<a class="headerlink" href="#how-to-free-things-with-cheri" title="Link to this heading">¶</a></h2>
<p>A very common design-pattern in encapsulating classes look something
like this:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>struct real_foo {
    struct foo foo;
    [some metadata about foo]
};

const struct foo *
Make_Foo([arguments])
{
    struct real_foo *rf;

rf = calloc(1, sizeof *rf);
    if (rf == NULL)
        return (rf);
    [fill in rf]
    return (&amp;rf-&gt;foo);
}

void
Free_Foo(const struct foo **ptr)
{
    const struct foo *fp;
    struct real_foo *rfp;

assert(ptr != NULL);
    fp = *ptr;
    assert(fp != NULL);
    *ptr = NULL;

rfp = (struct real_foo*)((uintptr_t)fp);
    [clean stuff up]
}
</pre></div>
</div>
<p>We pass a <code class="docutils literal notranslate"><span class="pre">**ptr</span></code> to <code class="docutils literal notranslate"><span class="pre">Free_Foo()</span></code>, another varnish style-pattern,
so we can NULL out the holding variable in the calling function,
to avoid a dangling pointer to the now destroyed object from
causing any kind of trouble later.</p>
<p>In the calling function this looks like:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>const struct foo *foo_ptr;
[…]
Free_Foo(&amp;foo_ptr);
</pre></div>
</div>
<p>If we use CHERI to make the foo truly <code class="docutils literal notranslate"><span class="pre">const</span></code> for the users of
the API, we cannot, as above, wash the <code class="docutils literal notranslate"><span class="pre">const</span></code> away with a trip through
<code class="docutils literal notranslate"><span class="pre">uintptr_t</span></code> and then write to the metadata.</p>
<p>The CHERI C/C++ manual, a terse academic tome, laconically mention that:</p>
<p><em>»Therefore, some additional work may be required to derive
a pointer to the allocation’s metadata via another global capability,
rather than the one that has been passed to free().«</em></p>
<p>Despite the understatement, I am very much in favour of this, because
this is <em>precisely</em> why my own
<a class="reference external" href="https://papers.freebsd.org/1998/phk-malloc/">phkmalloc</a>
became a big hit twenty years ago:  By firmly separating the metadata
from the allocated space, several new classes of mistakes using the
<code class="docutils literal notranslate"><span class="pre">malloc(3)</span></code> API could, and were, detected.</p>
<p>But this <em>is</em> going to be an embuggerance for CHERI users, because
with CHERI getting from one pointer to different one is actual work.</p>
<p>The only “proper” solution is to build some kind of datastructure:
List, tree, hash, DB2 database, pick any poison you prefer, and
search out the metadata pointer using the impotent pointer as key.
Given that CHERI pointers are huge, it may be a better idea to embed
a numeric index in the object and use that as the key.</p>
<p>An important benefit of this »additional work« is that if your
free-function gets passed a pointer to something else, you will
find out, because it is not in your data-structure.</p>
<p>It would be a good idea if CHERI came with a quality implementation
of “Find my other pointer”, so that nobody is forced to crack The
Art of Computer Programming open for this.</p>
<p>When the API is “fire-and-forget” like VSA, in the sense that there
is no metadata to clean up, we can leave the hard work to the
<code class="docutils literal notranslate"><span class="pre">malloc(3)</span></code> implementation.</p>
<p>Ever since <code class="docutils literal notranslate"><span class="pre">phkmalloc</span></code> no relevant implementation of <code class="docutils literal notranslate"><span class="pre">malloc(3)</span></code>
has dereferenced the freed pointer, in order to find the metadata
for the allocation.  Despite its non-const C prototype <code class="docutils literal notranslate"><span class="pre">free(3)</span></code>,
will therefore happily handle a <code class="docutils literal notranslate"><span class="pre">const</span></code> or even CHERIed read-only
pointer.</p>
<p>But you <em>will</em> have to scrub the <code class="docutils literal notranslate"><span class="pre">const</span></code> off with a <code class="docutils literal notranslate"><span class="pre">uintptr_t</span></code>
to satisfy the C-compiler:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>void
VSA_free(const struct suckaddr **vsap)
{
    const struct suckaddr *vsa;

AN(vsap);
    vsa = *vsap;
    *vsap = NULL;
    free((char *)(uintptr_t)vsa);
}
</pre></div>
</div>
<p>Or in varnish style:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>void
VSA_free(const struct suckaddr **vsap)
{
    const struct suckaddr *vsa;

TAKE_OBJ_NOTNULL(vsa, vsap, SUCKADDR_MAGIC);
    free(TRUST_ME(vsa));
}
</pre></div>
</div>
<p>Having been all over this code now, I have decided to constify <code class="docutils literal notranslate"><span class="pre">struct</span>
<span class="pre">suckaddr</span></code> in varnish, even without CHERI, it is sounder that way.</p>
<p>It is not bug, but CHERI made it a lot simpler and faster for me
to explore the consequences of this change, so I will forfeit
a score of “half a bug” to CHERI at this point.</p>
<p><em>/phk</em></p>
</section>
</section>

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    <h3><a href="../index.html">Table of Contents</a></h3>
    <ul>
<li><a class="reference internal" href="#">How Varnish met CHERI 6/N</a><ul>
<li><a class="reference internal" href="#varnish-socket-addresses">Varnish Socket Addresses</a></li>
<li><a class="reference internal" href="#how-to-free-things-with-cheri">How to free things with CHERI</a></li>
</ul>
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</ul>

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